1. Field of the Invention
The present invention relates in general to a transmission electron microscope. In particular, the invention concerns an improvement of a specimen illuminating or irradiating lens system composed of condenser lens and objective lens.
2. Description of the Prior Art
In a conventional high resolution electron microscope, a specimen is placed at a position Z.sub.o in a gap defined by pole pieces of an objective lens so that a magnetic field B.sub.z (Z) is produced around the position of the specimen in a bell-like distribution pattern, as can be seen from FIG. 1 of the accompanying drawings. In the hitherto known electron microscope of this type, the objective lens and the condenser lens are controlled independently from each other. Accordingly, inclination x.sub.o ' of an electron beam impinging on the specimen at a point x.sub.o is caused to vary when excitation of the objective lens is varied even if the excitation of the condenser lens is maintained constant. Further, variation in excitation of the objective lens will bring about variation in the position of convergence point of electron beam located at the side downstream of the specimen, even if excitation of the objective lens is so feeble that the intensity of magnetic field prevailing around the specimen can be neglected.
As the consequence, when the focussing excitation of the objective lens is varied in correspondence to variation in the position of the specimen such as tilting thereof, the position of the convergence point of the electron beam formed on the optical axis downstream of the specimen is displaced from the position of an objective aperture, involving reduction in the field of view or resulting in appreciable off-axis aberrations in a peripheral portion of the field of view due to remarkable variation in the direction in which the electron beam impinges on the specimen at a given point or area being observed. Thus, difficulty is encountered in producing an image of a satisfactory quality. Further, there arise such cases where the specimen disposed at a fixed position is to be observed in an under-focus or over-focus state under excitation of the objective lens which is different from the level employed for the observation in the just focus state. In these cases, magnification for observation in the under- or over-focus state will differ correspondingly from the magnification in the just focus state or the field of view is reduced by the objective aperture, to disadvantages. Such undesirable phenomena often occur in the observation of magnetic material or in which an image of a high contrast is to be produced with a relatively low magnification.